The present invention is concerned with a method for the casting of hollow precast units of concrete by means of slide-casting, wherein the concrete mix is extruded onto a base by using one or several shaping members that form a cavity and the mix is compacted by moving the shaping members. The invention is also concerned with a slide-casting machine for the casting of hollow precast units of concrete, the said apparatus comprising a deck plate, side walls, one or several feeding members for feeding the concrete mix, as well as one or several displacement shaping members for forming the cavities. The invention is suitable in particular for the production of prestressed hollow slabs. It may also be applied to the manufacture of hollow slabs of reinforced concrete.
In the prior art, several slide-casting machines for producing hollow slabs are known. These prior art machines operate on a similar principle, wherein the concrete mix is extruded in the machine by means of spiral screws. One of such solutions is described in the U.S. Pat. No. 4,046,848. The machine runs along rails placed on a base. The spiral screw is conical, the cone having a shape becoming wider towards the final end of the machine, whereby compacting of the concrete is also achieved.
As an immediate extension of the spiral screw, there is a shaping member, a so-called cavity mandrel, which is vibrated by means of a vibrator fitted inside the mandrel. The frequency of vibration is about 150 to 250 c/s. Moreover, a vibrator beam fitted in the deck part of the machine is vibrated, whereby the vibration of the cavity mandrels together with the surface vibration at the top of the machine produce the ultimate compacting of the concrete.
The cavity mandrel is followed by a so-called follower tube, whose function is to support the wall of the cavity at the final end of the machine.
Drawbacks of the cavity mandrel are the strong noise (more than 85 dBA) resulting from the high frequency of vibration, high requirement of power, and the low efficiency of the vibrating power used for the vibration.
From the Finnish patent publications Nos. 64,072 and 64,073, a method is known for compacting of concrete mix so that, instead of vibration, shearing forces are applied to the mix for the purpose of compacting the mix. The shearing forces are produced by moving two opposite walls of the mould back and forth in the same direction.
The invention provides a novel method and apparatus for producing hollow, pre-cast units of concrete that overcome the noise, power and efficiency drawbacks of the prior art slide casting machines. The method in accordance with the invention is characterized in that shaping members are used whose outer face is provided with a projection or projections, or a projection or projections are formed at the outer face of the shaping members from time to time, and the locations of the projections are changed relative the longitudinal axis of the shaping members, whereby the shaping members produce forces which compress the mix in the surrounding mix&lt;i.e. forces that compact the concrete. The slide-casting machine in accordance with the invention is characterized in that the outer face of the shaping members is provided with a projection or projections, or a projection or projections can be formed at the said outer face.
The projections on the shaping members can be produced so that shaping members are used whose cross-sectional shape differs from a circle. Alternatively, it is possible to use shaping members in which the shape of the outer face is modified. Herein, a projection is understood as meaning any projection whatsoever that projects from the circumference of a circle in the cross-section of the shaping member or that projects from a straight line in a longitudinal section. A projection in the cross-section changes its location by circulating around the centre part of the cross-section. A projection in the longitudinal section changes its position relative the longitudinal axis by moving axially and/or radially.
When the projections on the shaping members move, the shape of the cross section and/or longitudinal section of the space surrounding the shaping member is changed. The shaping members apply shearing forces to the mix, which forces make the aggregate particles in the mix seek new positions, whereby the mix is, at the same time, compacted.
When the compacting process produced by means of the method of the present invention is compared with the prior-art vibration compacting, it can be ascertained that in vibration compacting the frequency of the movement is high whereas the amplitude is little. On the contrary, in the method of the present invention, the frequency is relatively low whereas the amplitude is larger.
Owing to improved efficiency, the compacting process produced in accordance with the present invention is considerably more efficient in relation to the compacting energy used, compared with the compacting by vibration, while the noise level is, at the same time, significantly lower.